Kaolin is a naturally occurring, relatively fine, white clay which may be generally described as a hydrated aluminum silicate. After purification and beneficiation, kaolin clay is widely used as a filler and pigment in various materials, such as rubber and resins, and in various coatings, such as paints and coatings for paper.
As a pigment in paper coatings, kaolin clay functions to enhance the color, brightness, gloss, smoothness, printability and opacity of the coated paper.
Generally, for a kaolin clay to be of commercial value, all pigmentary impurity minerals (such as iron oxides) must be substantially removed. Processes typically used to remove these coloring impurities are high intensity magnetic separation, froth flotation, selective flocculation and chemical leaching. To be of commercial value, a kaolin clay must have certain desirable optical properties either in its natural state or as a beneficiated product. These properties include brightness, which is a measure of the amount of light reflected by a mineral pigment, and color which is generally measured using the Hunter L-, a-, and b-value system.
In the Hunter system, L-value provides a measure of lightness where whiteness is positive and blackness is negative, a -value provides a measure of redness where redness is positive and greenness is negative and b-value is a measure of yellowness where yellowness is positive and blueness is negative. In general, for paper coating applications, higher brightness clays are most desirable. As far as color is concerned, clay products having high L-, low a- and low b-values are most desirable.
A significant portion of the crude kaolin clays mined in Georgia are yellow tinted due largely to the presence of very small amounts of goethite (FeOOH). Typically, goethite is associated with kaolinite, the primary clay mineral comprising kaolin, or goethite may be found in association with titanium dioxide minerals such as anatase and rutile.
Two types of clay are mined in Georgia. One type is referred to as soft kaolin. This clay type occurs in Cretaceous age strata and is, in general, characterized by a low total iron content, low total titanium content, relatively coarse particle size and well-ordered crystallites.
The second type is referred to as hard kaolin. This clay type occurs in Tertiary age strata and contains more iron and titanium than the soft kaolins. Not only do the hard kaolin clays contain more iron than the soft kaolins, but this excess iron is found in the kaolinite structure. Another distinguishing feature is that the hard kaolin clays have a relatively fine particle size. Both the hard and soft kaolins can contain goethite which causes a yellow discoloration.
As is well known in the kaolin industry, many of these yellow-discolored clays do not respond to the standard beneficiation methods used for the removal of iron oxide minerals. This lack of response is due to the fact that goethite is less soluble, under typical reductive leaching conditions, than the other iron oxide minerals that are commonly found in kaolin deposits, such as hematite (Fe.sub.2 O.sub.3). Because these clays are visibly yellow and because they exhibit little or no color improvement in response to reductive leaching (when leached with the levels of sodium dithionite typically used in the kaolin industry), or any other standard beneficiation technique, they are often not considered of commercial quality.
One approach to upgrading these high b-value crude clays is to selectively remove the goethite. Because much of the goethite is submicron in particle size, the removal of goethite using particle size fractionation is not practical. Goethite also resists reductive leaching and is not substantially removed by magnetic separation. Froth flotation removes some goethite; however, this is a costly process.
Therefore, a need exists in the kaolin clay industry for a process which will effectively improve the color and brightness of kaolin clays and other materials which are discolored by the presence of goethite mineral impurities.